1. Field of the Invention
The present invention relates to an organic functional thin film and a process for the fabrication thereof. The organic functional thin film according to the present invention can be advantageously used as a non-linear material and organic semiconductor material in particular.
2. Description of the Related Art
As is widely known, non-linear optical materials are materials which demonstrate second- or third-order non-linear optical effects in the presence of an applied voltage or a strong electric field of laser light. In addition, in terms of phenomena, as such materials demonstrate functions of numerous devices such as optical frequency conversion, optical modulation, optical switching, optical memory and optical amplification, such materials have attracted considerable attention as fundamental materials in fields such as optical communications, optical interconnections and optical computers. Typical non-linear optical materials of the prior art are materials of inorganic systems, with widely known examples of such including crystals of KD.sub.2 PO.sub.4 (KDP), LiNbO.sub.3, KNbO.sub.3 and LiTaO.sub.3. Development and research of organic non-linear optical materials came to be actively conducted starting around 1983. Representative examples of such optical non-linear optical materials include MNA, DAN, MNMA and NPP. More recently, organic non-linear organic materials have appeared that have attracted the attention of the industry featuring electro-optical effects 10-100 times greater than that of LiNbO.sub.3 (usually referred to as LN) while also being able to be easily formed into a thin film.
Pendant-attached polymers are one type of organic non-linear optical material that is expected to be actively used as a material for electro-optic devices. Pendant-attached polymers are polymers in which non-linear optical molecules are introduced to the main chain of the polymer in the form of pendant groups (side chain groups). These are normally fabricated by forming into a film by spin coating. In other words, pendant-attached addition polymers are fabricated by forming film 7 of a polymer containing non-linear optical molecules (process A), and then applying a voltage between electrodes arranged above and below film 7, rotating those molecules and performing poling processing to orient the molecules in film 7 (process B) as is indicated schematically in FIG. 48. In process B, poling process can be performed by generating a charge on the film by corona discharge without forming the upper electrode. As is indicated in FIG. 48 (C), polymer film 7 exhibiting non-linear properties is obtained in this manner. However, in the case of the film formation method of the prior art, the characteristics of the resulting film are low due to being unable to attain a high level of orientation caused by thermal disturbance. In addition, due to considerable relaxation degradation resulting from the initial stable molecular state being random, decreases in characteristics over time are remarkable. Moreover, since this film formation method achieves orientation starting from a random molecular state, in the case of attempting to lengthen the size of the non-linear molecules and improve non-linear characteristics, the non-linear molecules are subjected to greater resistance with respect to rotation by the surrounding polymer. Since the molecule does not rotate satisfactorily, the orientation factor becomes excessively low thus allowing only low characteristics to be obtained similar to the case described previously.
Recently, a technique has been reported by the IBM Corporation in which an organic non-linear optical material is fabricated by mixing epoxy monomers and molecules containing amino groups followed by bonding and polymerization (R. J. Twieg et al., Preprints of CGOM (1989) V-05; D. Jungbauer el al., Appl. Phys. Lett. 56, 2610 (1990)). In this method, relaxation degradation is suppressed by formation of a network structure. Despite this, in the case of applying this method, film formation is performed by a coating process such as spin coating, thus resulting in problems including control of film thickness and film uniformity. In addition, the method also has problems that must be solved including unavoidable mixing of residual solvent and impurity molecules, and inadequate molecular orientation due to having to perform poling processing following film formation.
Moreover, organic functional thin films, which include non-linear optical materials, are normally fabricated using methods such as spin coating or dip coating. However, these methods also have problems that must be solved including difficulty in fabricating multi-functional and highly functional films since the hyperfine structure within the organic film that is obtained cannot be controlled.
Conjugate polymers are one type of material considered to be promising for use as organic functional materials such as non-linear optical materials and organic semiconductor materials. In order to improve the performance of such materials, it is necessary to develop long, conjugated .pi.-electron systems having minimal disorder. In addition, it is also necessary to selectively add donor or acceptor groups to conjugate polymers, perform doping or control the length of the conjugate chain. Various types of polymers have been developed as conjugate polymers, examples of which include polydiacetylene and polyphenylene-vinylene. However, in the case of the former, the obtaining of a high-performance film with minimal light-scattering has been difficult, and film formation has been difficult using MLD (molecular layer deposition). With respect to the latter, in addition to film formation being difficult using MLD, difficulty in controlling conjugate chain length, and difficulty in controlling donor groups and acceptor groups are involved.
On the other hand, it is known that double bonds (azomethin bonds) are formed by reaction of --CHO groups and --NH.sub.2 groups as indicated in the following formula. ##STR1## For example, Iijima et al. successfully formed a polymer thin film by means of deposition polymerization using the above-mentioned reaction (Nikkei New Materials, December 11, 1989, pp. 93-101).